In recent years, an IP telephone system configured to transmit/receive an image or voice as packet data bidirectional through an IP network in real time has become widespread. In this IP telephone system, a plurality of IP telephone terminals are accommodated in a private network such as a Local Area Network (LAN) or the like, the private network is connected to a global network such as a public network, Internet or the like through a router, and the router enables multimedia information communication between IP telephone terminals or between an IP telephone terminal and global network by carrying out address translation or the like.
Incidentally, a Network Address Translator (NAT) function is incorporated in the above system. The NAT function implies a function of translating a private address into a global address to transmit the global address as a transmission source IP address by using an IP address previously assigned for a connection purpose, and translating a global address of a destination IP address supplied by reception into a private address.
Incidentally, in the above system, at the communication time, when an inquiry is made about information on a voice channel used by a terminal on the control channel, an IP address and port number in a header area of the response packet are translated, whereas an IP address and port number in a data area of the packet, and used as a voice port are not translated by the NAT router. As a result of this, when a terminal which has received this packet tries to return voice data to the transmission source of the packet by using the IP address and port number in the data area of the packet, the terminal cannot correctly designate the destination, and it becomes substantially impossible to carry out communication.
The reason why the IP address and port number in the data area of the packet are not translated is that the fundamental function of the router is to translate an IP address and port number in the IP header in the packet.
On the other hand, as a network apparatus configured to translate IP addresses and port numbers in the areas including the data area when data is to be transferred, there is an application layer gateway (ALG). The ALG is compatible with a specific protocol such as SIP (Session Initiation Protocol), and translates, for example, an IP address and port number in a data area of a SIP packet.
However, the ALG makes only specific protocols such as the SIP the targets of address translation, and is expensive, and hence there is the problem that the cost burden is too heavy for introduction thereof into a small office.
It should be noted that as a relevant technique, a telephone exchange system in which a SIP telephone apparatus and router are connected to a LAN, and a telephone under the SIP telephone apparatus communicates with an SIP telephone connected to an ISP network through the router and SIP telephone apparatus is proposed (for example, Jpn. Pat. Appln. KOKAI Publication No. 2008-172552).
Incidentally, the telephone exchange system described above is premised on peer-to-peer communication, and is configured in such a manner that in order to accommodate a plurality of SIP external line trunks, a SIP wait port number is assigned to each carrier, and the SIP wait port number, and an IP address of the router configured to carry out routing toward the SIP carrier are correlated with each other so that SIP message communication between the carrier and IP exchange apparatus can be divided into communications for carriers.
On the other hand, although a system in which an IP voice packet is guided from an IP telephone in an external network to an NAT router, and is brought to an IP telephone of a communication partner by a port forwarding function of the NAT router is also conceivable, forwarding setting is required for all of IP telephones placed on the local network with respect to a forwarding table of the NAT router.
Further, although a system in which by providing an IP telephone apparatus and media gateway apparatus on a global network, an IP voice packet is passed from the inside of an NAT router to the outside there through, the IP voice packet is drawn into a media gateway, and the voice is relayed, whereby NAT router traversal is provided is also conceivable, it is necessary to place an IP telephone apparatus on the global network, and it is also necessary to prepare a global IP address.